The present invention relates to an apparatus and method for optical spectroscopy.
Optical spectrometers can be divided into dispersive or diffractive spectrometers and Fourier transform spectrometers.
Dispersive (prismatic) spectrometers or diffractive (grating) spectrometers break down the incident light ray into its spectral components by the wavelength dependency of an angle of diffraction or of an angle of reflection. The different spectral components are thereby spatially separated and the spectral component to be determined can be selected (monochromator). The recording of a spectrum then takes place with the aid of moving parts by the different spectral components being successively selected and measured.
Monochromators are most frequently used which have a Czerny-Turner ray path, i.e. which have a rotatable plano grating between an entrance gap and an exit gap and mutually independent collimator mirrors or collector mirrors.
The development of spatially resolving detectors (CCDs, diode arrays) now allows the simultaneous measurement of all spectral components by a separate element of the detector being provided for each spectral component. Such an arrangement does not require any moving parts and uses the available incident light substantially more efficiently.
Modern instruments use, for example, a holographic optical grating which can image an entrance gap directly onto a diode array with suitable spectral dispersion.
Fourier transform spectrometers are based on an interferometer in which the difference in the optical path lengths of the part rays brought into interference can be set with high precision. The spectrum can be determined by Fourier transformation from a measurement of the interference signal over a suitable range of path length differences.
Instruments are as a rule set up in the manner of a Michelson interferometer. However, above all the mechanical components for the setting of the optical path lengths by displaceable mirrors or tiltable mirror pairs are technically demanding here. The possible performance capability of dispersive or diffractive spectrometers depends on certain parameters, in particular on the dimensions of the entrance or exit gap, on the focal length and the aperture of the imaging elements and on the properties of the dispersive or diffractive element itself. Modern instruments almost reach these physically set boundaries.
The possible performance capability of Fourier transform spectrometers is accordingly determined by certain parameters and, here, in particular by the distance and the step width for the variation of the optical path lengths. The performance capability of Fourier transform spectrometers exceeds the possibility of dispersive or diffractive spectrometers by a long way.
Fourier transform spectrometers can also almost reach the physical boundaries of their performance capability, but the technical effort is very high in such a case. As Fourier transform spectrometers are based on an interferometer, all optical components, and in particular also the moving parts, must be produced and positioned with a precision of fractions of the wavelengths to be measured.